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Abstract

We demonstrate for the first time transmission of 54 Gbit/s and 48 Gbit/s over 44 km and 150 km, respectively, utilizing an optical bandwidth of only 3 GHz. We used polarization division multiplexed 512QAM and 256QAM modulation formats in combination with Nyquist pulse shaping having virtually zero roll-off. The resulting spectral efficiencies range up to 18 bit/s/Hz and 16 bit/s/Hz, respectively. Taking into account the overhead required for forward error correction, the occupied signal bandwidth corresponds to net spectral efficiencies of 14.4 bit/s/Hz and 15 bit/s/Hz, which could be achieved in a wavelength division multiplexed network without spectral guard bands.

Figures (4)

Schematic of elementary impulse and corresponding spectrum along with signal spectrum in one polarization only. (a) sinc-shaped Nyquist impulse and rectangular spectrum. (b) Power spectrum of sinc-shaped Nyquist-pulses belonging to a random sequence of constellation points, measured with a coherent receiver and in one polarization. The 14 dB signal bandwidth is 3 GHz for a symbol rate of 3 GBd. The noise floor is due to signal quantization. The out-of-band suppression is more than 36dB.

Transmission setup. (a) An optical transmitter based on an Agilent arbitrary waveform generator (AWG) generates, with the help of a narrow-linewidth fiber laser (laser 1), a variety of Nyquist sinc-pulses. The inset shows a schematic eye-diagram for a two-level zero roll-off Nyquist signal. The optical signal is split, de-correlated, and recombined in orthogonal polarizations to emulate polarization division multiplexing (PDM). Several fiber spans are inserted in-between two EDFAs before the signal is received by an Agilent optical modulation analyzer (OMA) using a second narrow-linewidth fiber laser (laser 2). (b) Several ULAF based transmission links that were used for our experiments. (c) Several SSMF based transmission links that were used for our experiments.

Experimental results for transmitted 256QAM and 512QAM signals. The first row shows the measured EVM of the two polarizations transmitted over a SSMF (red), an ULAF (blue), and back-to-back (b2b) (black). Limits of BER suitable for error-free transmission employing state-of-the-art FEC algorithms are indicated by dashed horizontal lines. (a) EVM performance of Nyquist shaped PDM-256QAM data for different fiber spans. (b) EVM performance of sinc-shaped PDM-512QAM data for different fiber spans. (c) Constellation diagrams for PDM-256QAM and (d) PDM-512QAM signals. Both polarizations are plotted within a single constellation diagram. The color coding refers to the type of fiber used for transmission.

Schematic and measured probability density functions (PDF) approximated by histograms of measured error vectors. (a) PDF of two neighboring symbols (blue and red) along the inphase axis for the real part of the electric field, along with their bell-shaped PDF of the error vectors. For non-data aided reception, the required decision threshold could lead to false symbol assignments. For BER ≤ 10−2 this effect is negligible [24]. (b) Histograms approximating a representative PDF both for real and imaginary part of all normalized error vectors of a 256QAM constellation diagram. Upper row: Back-to-back (b2b) transmission, lower row: 150 km ULAF. (c) Histograms approximating a representative PDF both for real and imaginary part of all normalized error vectors of a 512QAM constellation diagram. Upper row: Back-to-back (b2b) transmission, lower row: 44 km ULAF.